A 10 l-capacity galss-linked reaction vessel (with stirrer) was charged with 2 l of methyl isobutyl ketone warmed at 28° C. in advance, in which was dissolved 10 g of maridomycin III. To the solution was then added 2 l of a tris(hydroxymethyl)aminomethane-maleate buffer solution (0.2M. pH 7.0) dissolving 500 mg of carboxyl esterase obtained in Reference Example 1. To the solvent layer was added 20 ml of propionic anhydride, and the reaction was allowed to proceed for one hour under stirring. The reaction solution was shifted to a 10 l capacity separating funnel, and thus the aqueous phase was removed. To the remainder was added 500 ml of an aqueous solution of sodium bicarbonate (2%) (w/v). The mixture was sufficiently stirred, followed by removing the aqueous portion. Washing with an aqueous solution of sodium bicarbonate was repeated once more, followed by washing with 2 l of distilled water twice in the separating funnel. The organic phase was shifted to a 3 l-capacity conical flask, to which was added 100 g of anhydrous sodium sulfate, and the mixture was left standing for two hours. The anhydrous sodium sulfate was filtered off by using filter paper, and the reaction solution was concentrated to dryness to obtain 10 g of 9-propionyl maridomycin III as crude crystals.
